1. Field of the Invention
The invention relates to a method of fabrication of preforms for the resin transfer molding process. A preform is the fiberglass reinforcement package that has the shape of the desired composite part and has the correct glass type (chopped or continuous) and glass fiber orientation (random or directed). The use of preforms in this process advantageously reduces mold cycle times due to the more rapid positioning of the preformed reinforcement material in the mold. Fewer parts are rejected because the actual distribution of the reinforcement in the mold may be checked before molding.
2. Discussion of the Prior Art
There are two basic approaches for fabricating preforms for the resin transfer molding process. In the first approach, fiberglass reinforcement is deposited directly on a screen. One example of deposition methods in this category is chopped glass spray-up, or directed fiber placement. Such an approach may be studied in the article entitled "Preforming for Liquid Composite Molding" by E. P. Carley et al published in Proceedings of the 44th Annual Conference of the Society of the Plastic Industry, Feb., 1989 session 10-B.
When chopped glass strands are sprayed to form a preform, consistent production results are difficult to achieve because of the inclusion of a manual operation in the process. Local variations in fiber deposition up to 40% have been noted. Other difficulties in making preforms symmetrically balanced with regard to their vertical rotational axis have been encountered. Additionally, the minimum radius of curvature which can be obtained using chopped strands and the air deposition process is relatively high due to the "springy" nature of the chopped strands.
The second basic approach for fabricating preforms requires fabric as the starting point for reinforcement, as noted in "Preforming of Unifilo" by L. Pomini of Vetrotex St. Gobain, published in the Reinforced Plastics Congress 13th, 1986 by the British Plastics Federation Publication N293, London, England, and in "Preforming Continuous Filament Glass Mat" by Mark Hickling of Technical Services Vetrotex U.K. Ltd., Wallingford, Oxon, (available NTIS). The fabric may consist primarily of random or continuous fibers.
Regardless of the type of fabric used, it is necessary to cut the fabric in a pattern that will cover the mold surface with the correct fiber orientation.
Once the fabric is cut, it has to be formed to the shape of the mold surface. In hand-layup operations, the fabric is usually hand-fitted to the surface in a relatively labor-intensive manner. Other methods require that the fabric be tensioned in a machine frame while a set of matched-cavity dies form the fabric to the desired shape. (The design of such a machine may be studied, for example, in the sales literature of EMC Machinery, P.O. Box 9800, Fort Worth, Texas). This method generates 20 to 25% waste of preform material due to the necessary trimming of the edge of the fabric to the edge of the mold. This wastage is due to the excess material held by the frame beyond that required to reach the outer boundaries of the part. This method works best with random continuous glass mat.
Use of the machine frame is limited either to forming the fabric to a mold surface having simple contours, accepting a large number of wrinkles in the final shaped preform, or rejecting a large number of preforms due to unacceptable fabric distortion or tearing.
In applications where tight control must be maintained on fabric thickness it is desirable to replace the chopped glass spray-up fiber deposition which has thickness variations with fabric having uniform thickness. Additionally, undesirable channels are not formed through fabric during the period that resin is flooded into the mold and through the fabric, because the fabric is not preferentially oriented as found in sheet molding compounds. Consequently, mechanical properties and surface finish are improved over sheet molding compound and sprayed-up fiber depositions by the use of fabric.
As the severity of the edge and contour topography within a mold increases, the fabric tends to wrinkle when forced around the edges and contours such that unwanted quantities of the fabric "bulge", fold over, wrinkle, and generally displace from a substantially planar position adjacent the mold surface. Current technology therefore limits the use of preform fabrics to use within molds having gentle contours.
Superplastic aluminum diaphragms have been used to form metals and, more recently, thermoplastic resin impregnated carbon fiber fabrics. Aluminum diaphragms were required in the case of the thermoplastic impregnated fabric because of the high temperature (approximately 700.degree. F.) at which the thermoplastic resin softened to permit forming to occur. The diaphragms can be used only once and, as a result, the process is quite expensive and does not lend itself to desired preform mass production requirements.
More recently, the superplastic aluminum diaphragms have been replaced with a polymeric diaphragm, as noted in "Composites Update", University of Delaware, Autumn 1987. This diaphragm, like the superplastic aluminum diaphragm, is not reusable.
A method needs be developed therefore that allows the use of fabric within a mold, wherein the mold has accentuated edge and contour sections. Such a method should lend itself to the mass production of the preforms in an economical manner. The preform material during the forming process should not wrinkle nor tear, and preferably the fabric should be "cut to fit", wherein expensive trimming of the fabric edges is eliminated, after the preform material is shaped.